1. Field of the Invention
This invention is in the field of fuel transfer pumps as used, for example, in inflight refueling of aircraft.
2. Background of the Invention
Relatively high flow fuel transfer pumps are needed in several applications throughout industry. The invention described herein is primarily designed to overcome one particular problem in one particular application, pumps used to transfer fuel between aircraft in flight. The discussion will be limited to the problem of how to avoid ignition of fuel vapors by the pumping machinery, even though other advantages of the invention such as long unattended service life may prove useful in other fields or for other applications.
Combustible vapors or mixtures of fuel vapor and air occur in the ullage space of jet aircraft fuel tanks at ambient temperatures (as well as elsewhere). The pump used to empty the tank or other vessel is inevitably and customarily immersed in the liquid, but when the fuel is almost drained or completely drained, the pump and its motor are in direct contact with gases which can be ignited. The ensuing heat release or explosion is extremely hazardous so the prior art contains a number of inventions and practices to avoid ignition. My invention carries the art a step beyond the existing art by ensuring the absence of hot metal or carbon in the pump component itself.
Common practice is to drive a centrifugal pump with a hydraulic motor driven by about 3000 pounds per square inch (psi) hydraulic fluid and exhausting into a reservoir at about 100 psi. A multiple-piston type hydraulic motor which can be fully sealed is used. To prevent hydraulic oil which leaks slowly past the pistons from pressurizing the case, a direct drain port is provided to return "case leakage" to the reservoir. This case drain is in parallel to the reservoir-return line from the working pistons, and flows a few cubic inches of hydraulic oil per minute. Thus, the drive motor for the centrifugal pump presents no ignition hazard for the pumped fuel vapor. It is inherently safe because the motor will not operate without case leakage--the pistons see no pressure differential.
Present-day practice is to use precision graphite sleeve bearings and thrust bearings to guide the impeller shaft to hold the impeller itself (which runs with small clearance to the pump housing in places) spinning on the proper axis. In the event the impeller spin axis drifts sideways toward the housing "wear rings" are provided for the closely fit parts which, for pump efficiency, must limit recirculation leakage of the fuel at higher-than-inlet pressure back to inlet pressure. Pump efficiency dictates close tolerances on the impeller-to-housing gaps; pump life dictates the reverse.
When the prior-art pumps are run dry, lubrication of the graphite sleeve bearings (which are usually lubricated by fuel) ceases to exist and the graphite wears rapidly. Not only does this reduce pump life by requiring the wear rings to guide the spinning impeller, it heats the impeller, wear rings, the now-unlubricated bearings, and housing--occasionally to about 400.degree. F. which is high enough to ignite the fuel vapor-air mixture, allowing flame to propagate throughout the vessel. More than one fatal aircraft crash has been attributed to exactly the occurrence described above. The age of the pumps prior to these occurrences is not known to me, but even a new graphite sleeve bearing can wear or disintegrate and generate heat rapidly when run dry.